The invention relates to the field of catheter delivered stents, and in particular to a catheter slip sensor.
Slippage or loosening of a stent on the delivery catheter impairs the accuracy of stent placement in up to 10% of all procedures, yet current stent delivery technologies do not inform the operator of slip until after the stent is irreversibly expanded. This is especially important given the rapidly increasing popularity of many stent designs (there were over 200,000 endovascular stents implanted in the U.S. in 1996) and the fact that clinical failure of a stent implantation may occur with placement errors as small as a few millimeters.
An endovascular stent is a hollow, expandable tubular structure that is mounted over a catheter and is threaded through a hemostatic valve into the vasculature. Once positioned, it is expanded by either inflating a luminally-mounted balloon or by retracting a restraining sheath that permits the elastic stent to spring open.
Slippage can occur as the stent passes into the artery through the hemostatic valve, or more commonly, while navigating tortuous vasculature. This second cause of slip occurs in a two-stage process: leading edge separation and edge snare. As the catheter is pushed through a sharp curvature, it bends more abruptly than the stent. This causes a separation between the catheter and the distal end of the stent along the outer edge of the curvature. The leading edge of the stent now protrudes beyond the catheter profile, and becomes ensnared in the arterial bend.
Stent slip on the delivery catheter is difficult to directly measure. The majority of endovascular stent designs, including the two currently approved by the FDA, are only slightly radio-opaque, making stent placement difficult. The majority of stenting systems crimp the stent over radio-opaque markers on the delivery catheter to provide the operator with indirect evidence of the stent location under fluoroscopy. This indirect method is inaccurate and misleading if the stent slips. The operator, unaware of the displacement, may attempt to deploy the stent once the catheter markings are positioned within the stenosed region, resulting in inaccurate placement, incomplete expansion, or total nondeployment of the stent. The success of the stent positioning can be ascertained only after deployment, through indirect angiographic evidence of the flow patterns of radio-opaque dye though the stented vessel.
There are more serious ramifications of the operator being unaware of a displaced stent than inaccurate placement. If the stent begins to loosen on its delivery catheter it can slide off entirely once its protective sheath is retracted. If the stent dislodges in the distal direction it may enter into the circulation, requiring emergency surgery for retrieval. If it slides too far proximally it will not deploy and then can later slip over the balloon in the distal direction as the balloon is being retracted into the guide catheter. These issues could be avoided if the operator could sense displacement of the stent along the delivery catheter, as the operator could then opt to either retrieve and recrimp the stent before retracting the guide catheter or deploy the stent immediately before greater risk of stent displacement is encountered.
The danger of stent movement on the guide catheter is evinced by the variety of means that have been proposed to negate it. As discussed below, none have proved entirely effective. There is a device that detects the presence or absence of the distal region of a stent against the catheter, but it cannot measure the position of the stent along the catheter nor detect proximal slippage. The system we propose is capable of detecting both a proximal and distal dislodgment of the stent as well as measure the relative position of the stent against the catheter. In one embodiment it can further provide information about localized regions of detachment, such as occur just prior to axial slip when advancing the catheter past a tight arterial bend. We have constructed a prototype, as detailed below, and have demonstrated its feasibility.